The present invention relates to an electrochemical energy store and to a method for balancing a multiplicity of strings of electrochemical storage modules of an electrochemical energy store connected in parallel with one another. Electrochemical energy stores consist of individual structural units (“modules”) in order to be able to provide the high currents which are required for some applications and which can no longer be provided by a single module. For said application it is necessary to connect modules in parallel. This is in turn not readily possible with standard modules since compensation currents that can lead to undesired operating states can flow between the modules. The modules of present rechargeable battery cells typically contain fluids based on lithium chemistry. These cells are sensitive both to overcharging and to deep discharge. An overcharging above approximately 4.2 V per cell or module results in exothermic processes that can lead to the destruction of the cell. A deep discharge below approximately 2.5 V per cell or module leads to permanent degradation of the energy storage and current carrying capacity of the cell. If cells are connected in series, then they are charged and discharged only jointly. In other words, the charge changes of each cell of the string are identical. Differences in the cell constitution then have the effect that the state of charge (SOC) of the cells is no longer identical with increasing use. This may have the effect that during charging individual cells may be overcharged, while other cells are not yet fully charged. The latter alternative entails the risk that during discharging individual cells may be subjected to deep discharge, even though other cells are not yet fully discharged. For this reason, the state of the rechargeable battery cells is monitored permanently and charging or discharging is possibly prevented. A process of charge equalization—also referred to as “balancing”—between the cells of a rechargeable battery pack (“electrochemical energy store”) pursues the aim of all the cells having the same SOC. The balancing of the cells within a module is ensured by an integrated measuring and balancing circuit designed for this purpose.
Moreover, the prior art discloses so-called UniBB (universal buck-boost) modules comprising an electrochemical energy store (e.g. lithium ion based or lithium polymer based) and an electrical circuit by means of which they can realize different terminal characteristics. By way of example, a UniBB module can be used both as voltage source and as current source. The electrical circuit within the UniBB module could also be referred to as “coupling unit” which contains an inductance. The construction and the functioning of a UniBB module is known for example from the prior art.
CN 102496970 discloses a balancing system for an electrochemical energy store of an electrically operated means of locomotion. The electrochemical energy store contains two storage strings connected in parallel with one another. Each of the storage strings contains individual storage cells. Via a resistance and a capacitance, an SOC value of the storage strings is determined and energy of individual cells is shifted to other cells having a lower SOC.
US 2011/025258 discloses an interconnected system of electrochemical storage cells in which balancing is carried out whilst avoiding overcharging or deep discharge of the cells. The document discloses a control unit that monitors the SOC of the cells and groups the cells into dischargeable and chargeable cells on the basis of their SOC. Depending on their group association the cells are charged either individually or jointly.
US 2009/208824 discloses an interconnected system of electrochemical storage cells which are connected in parallel with one another. The cells have a respective controller, which can be configured as a buck-boost controller. If it is recognized that the cells have different states of charge, the energy drawn from the individual cells is varied. This prevents different cells from having different states of charge for a long period of time.
It is an object of the present invention to improve balancing of electrochemical stores connected in parallel.